In an automobile field, for purposes of supporting driving, improving safety, and the like, various kinds of research and development have been made such as on an Adaptive Cruise Control (ACC) and pre-crash safety systems. The ACC and the pre-crash safety systems are techniques of capturing an image ahead of a vehicle with a stereo camera mounted on the vehicle, measuring a relative distance to a preceding vehicle from the captured image, and allowing the vehicle to follow the preceding vehicle or to brake predicting a collision against the preceding vehicle.
In a case where the relative distance to the preceding vehicle is measured from the image obtained with the stereo camera, the relative distance between the vehicles or the like is calculated by using parallax information of the same vehicle displayed on the images obtained with two imaging units (cameras). In recent years, downsizing of the stereo camera has been demanded in particular; however, there is a problem in that a decrease of a baseline length between the cameras constituting the stereo cameras results in a decrease of a detection distance. Accordingly, in a long distance region away from the vehicle, an image obtained with one of the cameras (monocular camera) constituting the stereo camera is used to detect a vehicle position from the image. The relative distance between the vehicles is calculated from width information (the number of pixels) on the detected image, an estimated vehicle width (for example, 1.7 m), and characteristics of the camera. In addition, a method for calculating the distance to the preceding vehicle is switched between a short distance region close to the vehicle and the long distance region away from the vehicle. In this way, accuracy of object detection around the vehicle is maintained (for example, PTL 1).